1. Field of the Invention
The present invention relates to a circuit for rapidly determining the largest difference among three binary numerical values and for allocating the maximum difference to one of a plurality of numerical ranges and for generating a control signal corresponding to that range. Circuits for accomplishing these results are particularly useful in two-dimensional differential pulse code modulation (DPCM) coding wherein a switchable quantizer and a quantizer controller are utilized to which the most recently calculated numerical value representing a picture element or pixel signal value as well as picture element signal values adjacent to the most recently calculated value are supplied via registers, and a comparison device and an extreme value control are provided for determining the greatest and smallest value of the adjacent picture element signal values. A subtraction means for forming the differences is utilized. Such a DPCM system is described for example, in the co-pending application of Alexander Starck, filed Aug. 19, 1984 and assigned U.S. Ser. No. 641,727, corresponding to German application No. P3331426.8, and assigned to the assignee of the present application.
A problem in the use of such devices is that the greatest difference between a plurality of numerical values must be determined when processing the data, particularly when coding television signals. In DPCM coding of television signals, a quantizer is controlled dependent upon the contrast of a picture element to be coded with respect to the picture elements surrounding the element to be coded. If only slight contrast between the element to be coded and the surrounding elements exists, such contrast generally referred to as "activity" in controlled DPCM coding, the coding then takes place in small steps, whereas larger quantizing steps are employed given greater activity. An improvement in the picture quality is thereby achieved. This procedure is explained in greater detail in the aforementioned patent application Ser. No. 641,727. Processing of the last picture element signal preceeding the picture element to be coded is critical with respect to time. All other picture element signals will already have been available to the coder or the decoder at the receiver for a longer time. As a result, preprocessing of those signals can already be undertaken, so that an arithmetic operation in which the last picture element signal value, referred to herein as value A, still must be undertaken.
This arithmetic operation takes the form of determining the differences between the signal value A and the two extrema, referred to herein as E and F, of the previously processed signal values. The maximum differences determined by the operational signs of these differences and is forwarded to other processing circuitry via multiplexers. Although conventional difference selecting circuits for determining the maximum difference generally contain only a few gate circuits, such gate circuits cause transit times which are unacceptably high given the higher processing speeds of which the remainder of the processing circuitry is capable.
After determining the greatest difference, this value is supplied to a threshold logic circuit which classifies the difference in one of several numerical ranges, and emits a control signal depending upon the classification. An additional transit time delay results from this circuit as well.